Dimmer circuits are commonly used to control power, in particular alternating current (AC) mains power, to a load, such as a light source. In one existing method, a light source can be dimmed using phase controlled dimming whereby power provided to the load is controlled by varying the amount of time that a switch connecting the load to a mains power source is conducting during a cycle of the AC (i.e. varying the duty time). Specifically, AC power to the load is switched ON and OFF during each half cycle of alternating current and the amount of dimming of the load is provided by the amount of ON time in relation to the OFF time for each half cycle.
Phase control dimmer circuits generally operate as trailing edge or leading edge dimmer circuits, and the two circuits are suited to different applications. In leading edge circuits, power is switched OFF at the beginning of each half cycle. In trailing edge circuits, power is switched OFF later in each half cycle (e.g. towards the end of each half cycle). Leading edge dimmer circuits are generally better suited to controlling power to inductive loads, such as small fan motors and iron core low voltage lighting transformers. Trailing edge dimmer circuits, on the other hand, are generally better suited to controlling power to capacitive loads, such as drivers for Light Emitting Diode (LED) lights.
Phase control dimmer circuits, however, can produce line conducted harmonics causing electromagnetic interference (EMI) emissions when switching ON and OFF power to the load—particularly, for instance, switching ON and OFF power to complex loads such as compact fluorescent lighting (CFL) and LED light drivers. More specifically, these dimmer circuits include a switching circuit and a switching control circuit for controlling delivery of AC power to the load by conducting power to the load in an ON state (conduction period) and not conducting power to the load in an OFF state (non-conduction period). During the OFF state of each half cycle of AC, power is available to the dimmer circuit for operation thereof.
In some exemplary prior art dimmer circuits, the switching control circuit includes a zero-crossing detection circuit configured to detect a zero crossing of the AC to define the conduction periods and non-conduction periods in an ideal dimmer circuit. In practice, however, many exemplary existing dimmer circuits (e.g. 2-wire trailing edge phase control dimmer circuits) exhibit half-cycle conduction period commencement several tenths of a millisecond prior to true zero-crossing; hence, step-voltage is applied to the load which can cause current pulses and EMI emissions especially with capacitive electronic load types such as LED or CFL light drivers.
The AC (line) power provided to the dimmer circuit in the non-conduction period is first rectified by a rectifier. The rectified dimmer voltage (e.g. rectified via full-wave rectifier) is of a pulse form normally having repetition rate equal to twice the line frequency. The rectification, however, produces parasitic capacitance and a relatively high zero-crossing voltage is necessary to mitigate filtering effects of the parasitic capacitance.
Thus, with some capacitive-input and low power-factor load types, the effect of a non-zero zero-crossing voltage threshold can cause a significant advancement of the conduction period commencement, especially when the exemplary dimmer circuit is operating at higher operating conduction angles. In these cases, the result for these load types can be a noticeable reduction in achievable brightness at a maximum dimmer setting, and/or the onset of asymmetry of non-conduction periods which can cause an undesirable effect of flickering of an LED light driven by the load. Both these effects are due to a reduced magnitude of dimmer voltage, in some cases resulting from the load type topology, in comparison to the corresponding line voltage caused by the advanced zero-crossing.
As described, one of the major causes of inaccurate zero-crossing detection can be attributed to the effects of component parasitic capacitance associated with the rectified dimmer voltage. Such capacitance acts to partially smooth the rectified dimmer voltage and causes some lagging phase-shift of the zero-crossing where the voltage minimums do not reach zero. Thus, in order to minimise these filtering effects, an exemplary prior art trailing edge phase control dimmer circuit requires lower (dissipative) impedance. This lower impedance, however, causes higher dimmer losses and thus is not a satisfactory solution to minimising the detrimental filtering effects.